This invention relates to sources that emit acoustic energy underwater. In particular, this invention relates to a sparker including electrodes and insulation for a pulse powered underwater sound source for minesweeping.
Currently, efforts are being made to develop new high speed, remote/autonomous minesweeping systems that will fit on small surface platforms. The acoustic technology chosen for this effort has concentrated on pulse powered sparkers where bursts of electrical current are discharged in the water by a sparker device that operates much like a spark plug in an automobile engine. Each discharge produces a bubble in the water much like detonation of a chemical explosive. The noise created by the chemical explosive or discharged bubble is primarily generated by the large pressure pulses emanating from the expansion and collapse of the bubble. The choice of the spark gap for the sparklers was driven by the need for low in-water drag during high speed transit and minesweeping operations, relatively good efficiency at low frequencies, and some capability to change the acoustic output to emulate the signature of a specific craft. While underwater discharges have been under investigation for several decades, most of the work involved single shot laboratory systems or systems that fire a burst of shots over a relatively short period of time, e.g. during a few seconds. The driving requirement for this needed design is that the sparker must be capable of operating over extended periods of time extending from tens of minutes to hours, without having to stop operation for adjustment or replacement. Accordingly, the performance of such a system is highly dependent on the spacing of the gap between electrodes and exposure of the positive electrode (anode) especially if the gap and/or the surface area of the anode exposed to the sea water becomes too large for the pulse power system to efficiently break down the water. Additionally, the operation of the desired system in terms of acoustic output had to be relatively consistent from spark to spark or from one group of sparks to another; the sparker had to be small in size to minimize drag; and the sparker had to be relatively cheap to fabricate and be easily installed.
Recently, efforts in developing a spark that can be reproduced over a long period have been mostly concerned with developing air-gap switches. Currently, the greatest use of underwater sparkers in a repetitive mode is in lithotripsy (a commercial medical application). However, the sparkers typically only lasted a few hundred shots which would amount to less that a minute's operation in the minesweeping application. Furthermore, the lithotripsy sparkers are used at much lower energies than those required for minesweeping so that these designs would not hold up during prolonged minesweeping. This is because the erosion of the sparkers typically scales with energy of discharge for a given system.
In the 1960's several underwater sparkers were developed for the oil exploration industry. These systems typically operated at a repetition rate almost 40 to 60 times less than that required for successful minesweeping. Moreover, the efficiency of these systems varied. One system with the best efficiency used a wire initiated discharge system that fed a small gauge wire across a pair of electrodes. Electric current then passed through the wire causing it to vaporize and create the plasma for the discharge. The other system used a single electrode which created a corona discharge at the tip of the electrode as current passed from the electrode to salt water that acted as the negative electrode. The efficiency of this system was typically poor but was compensated for by using large discharge energies.
In addition, other previous attempts by different groups to develop a long life sparker involved the use of point-to-point electrodes. Point to point electrodes are electrodes that face each other on an axis going through their centerline. In the first attempt the electrodes were hollow rods through which heavy gauge wire was fed. The wires were positioned a specific distance apart (.about.0.25 inches). The discharge occurred between the ends of the wires and resulted in the erosion of both wires. This caused the gap between the wires to grow and the center of the gap to move to one electrode's feeder rod since the erosion was different for the different rod polarities. The feeder mechanisms for the wires had to not only compensate for the enlarged gap but also for the shift in the gap center. Additionally, the erosion of the soft copper wire was not constant from shot to shot. To deal with these challenges a controller system was built that required monitoring the intergap resistance between shots. This was effected by the residue left in the water from the discharges. A two quadrant optical system also was needed to detect the discharge position. The resulting system was complex and difficult to properly adjust and, in fact, was never successfully demonstrated. In addition, the system had poor efficiency and could not break down water with considerably less salt content than sea water. This required the sparker to operate in a large fresh water chamber which also called for ancillary water conditioning systems. This made the system much too large and cumbersome.
In the second attempt to develop a long life sparker, the electrodes were thin-wall cylindrical stainless tubes which were mounted on an isolating substrate with the rods facing each other. The electrical connection to the cathode was made via a small rod that passed through the center of the isolating substrate that separated the electrodes. This configuration was thought to enhance the break down of the water. This configuration had better shot to shot performance in terms of efficiency and could break down sea water, but the erosion rates of the insulating substrate and the electrodes were high so that the sparker had a lifetime that was limited to within minutes. Bulkier electrodes were tried to increase the lifetime but the efficiency dropped off dramatically and the lifetime of the substrate was still an unsolved problem. Different materials were tried for the substrate but none gave performance close to what was needed.
A more recent attempt to develop a long life sparker began by investigating the configurations of other sparker systems, especially those mentioned above that had been developed for oil exploration. The wire initiated concept was of interest because of its high efficiency which is important for keeping the size of the overall system down. A test article was designed, fabricated and operated with some success. However, it was found that developing a reliable wire feed system that could feed the wire at the rates required for successful operation would be difficult at best and would require an unduly long time to perfect.
The concept of using a sparker having a single electrode that is referred to above was given renewed interest because of its inherent simplicity. This feature, it was thought, might facilitate developing a long-life sparker provided that the efficiency could be improved. Further experiments with the single electrode design indeed showed that the efficiency could be improved significantly with the optimum selection of circuit components; however, this improvement was still not sufficient to meet the minimum requirements needed for minesweeping over long periods with some margin. Additionally, the erosion of the electrode insulation was still problematic.
Thus, in accordance with this inventive concept, a need has been recognized in the state of the art for a cost-effective coaxial sparker for pulsed underwater sound sources that maintains a relatively constant gap between a cathode and a coextensive, coaxial anode separated by an insulator.